1. Field of the Invention
Embodiments of the present invention generally relate to making and detecting document verification indicators and, in particular, to optically encoding documents with spatially varying refractive index patterned indicators and detecting the patterned indicators using a bi-directional image capturing system.
2. Description of the Related Art
Physical watermarks or watermarking in the form of watermark labels and raised seals have long been used as a means to identify and authenticate documents. These identifying marks can be easily reproduced, however, so they offer little protection against most attempts at producing a fraudulent document. Making physical document verification robust and reliable is of great importance these days, especially in view of our nation's heightened security needs and the increased requirement for accurate identification.
Known systems have attempted to overcome the inherent issues with the aforementioned physical watermarking techniques. For example, known methods use markings outside the visible spectrum. In one example, there is a method of applying a watermark visible only with an infrared detector. In another example, there includes invisible ink applied to a document where the invisible ink is only detectable using an invisible ink detection system. However, these methods are only a small advancement over visible watermarks and minimal sophistication is needed to compromise the integrity of these “invisible” watermarks through the use of inexpensive and readily available infrared cameras, and the like.
In other known watermarking methods, holograms, intaglio printing, and zero-order micro-gratings have been used for document security. These techniques may be more difficult to copy than simple scanning but the patterns are visible by the unaided eye and can be duplicated with known image capturing systems and techniques.
Thus, there is a need for the ability to produce a secure, identifiable document verification indicator, such as a watermark, undetectable by the unaided eye, conventional detection systems such as infrared detection systems, and the like. There is also a need for a document verification indicator system that can include unique information in the patterned indicator to further verify, through the use of a detection system, that the document is authentic.